1. Field of the Invention
The present invention relates to a two-terminal nonlinear device. Such a device may be used, for instance, in a liquid crystal display apparatus, as a switching element.
2. Description of the Related Art
In recent years, liquid crystal display apparatuses have been widely used in various fields, such as an audio visual (AV) field and an office automation (OA) field. In particular, products of the low end are equipped with twisted nematic (TN), or super twisted nematic (STN) passive type liquid crystal display apparatuses, and products of high quality are equipped with active matrix type liquid crystal display apparatuses using thin film transistors (TFTs) which are three-terminal nonlinear devices.
The active matrix liquid crystal display apparatus is superior to a cathode ray tube (CRT) for its characteristics of color reproducibility, saving space, light weight, and lower power. Due to such characteristics, applications thereof have been rapidly developed. However, in the case of using the TFTs as switching elements, from 6 to 8 times or more fabrication processes of a thin film and a photolithography process are required for forming the TFTs, thus increasing the manufacturing costs. On the other hand, the liquid crystal display apparatus using two-terminal nonlinear devices as the switching elements is superior to the liquid crystal display apparatus using the TFTs for its savings in cost, and superior to a liquid crystal display apparatus of a passive type for its display quality. Thus, the liquid crystal display apparatus using two-terminal nonlinear devices has been rapidly developed.
As the above-mentioned two-terminal nonlinear device, two-terminal nonlinear devices of a Schottky diode type, a varistor type, and an MIM (metal-insulator-metal) type have conventionally been known. In recent years, two-terminal nonlinear devices of a D.sup.2 R (double diode plus reset) type and an organic ferroelectric thin film type have been widely studied. However, only the MIM and D.sup.2 R two-terminal nonlinear devices are in practical use. The MIM two-terminal nonlinear device (hereinafter, referred to as "the MIM device") includes upper and lower electrodes interposing an insulator therebetween. For example, the MIM device disclosed in Japanese Patent Publication Nos. 61-32673 and 61-32674, and U.S. Pat. No. 4,413,883 is explained. The lower electrode is formed of a thin Ta film on a substrate or a base coating film formed thereon. The insulator is formed by the anodization of the surface of the lower electrode. In this case, the insulator is a Ta.sub.2 O.sub.5 layer. The upper electrode of one of Ta, Cr, Ti and Al is formed thereon. The MIM device can be produced using less than one third of processes required for fabricating the TFT. Therefore, the MIM devices are mainly used in the liquid crystal display apparatus using the two-terminal nonlinear devices.
The liquid crystal display apparatus using the MIM devices includes an active matrix substrate on which MIM devices and pixel electrodes are formed and a counter substrate on which the wiring of an ITO transparent conductive film or the like, is formed in a stripe shape so as to cross the wiring provided on the active matrix substrate at right angles. The two substrates are attached to each other by pressure and heat, thereby fabricating a liquid crystal cell.
The liquid crystal cell is fabricated as follows:
First, an orientation film formed of polyimide type organic polymer is coated onto each of the active matrix substrate and the counter substrate, then subjected to a rubbing treatment so as to align liquid crystal molecules. Successively, a sealing agent is coated onto one substrate and a spacer is dispersed on the other substrate. The two substrates in this state are attached to each other and pressed by heat. After that, liquid crystal is injected between the substrates and the resulting substrates are sealed. In this way, the liquid crystal cell is fabricated.
In order to realize a display with high quality, it is required that the MIM device has a symmetrical curve of a current-voltage characteristic while a positive voltage and a negative voltage are applied to the lower electrode. Further, the MIM device capacity should be smaller than the liquid crystal capacity. The asymmetrical curve of the current-voltage characteristic causes the occurrence of a residual image on the display. There is a problem regarding crosstalk, in the case where the MIM device capacity is not small enough with respect to the liquid crystal capacity.
In order to prevent the residual image and crosstalk, various techniques have been studied. For example, an insulator is usually formed by anodizing a lower electrode. As described in Japanese Patent Publication No. 46-17267, anodization is a conventionally established method. An insulator having high through-put and superior productivity can be obtained by the anodization. Further, a technique in which the resistance of wiring is reduced in order to obtain a display apparatus of a larger size and high quality has been studied. For example, a thin Ta film used for the wiring is doped with nitrogen so as to reduce the specific resistance. In this case, it has been confirmed that the specific resistance of the Ta thin film can be reduced down to 40-100 .mu..OMEGA.cm.
Japanese Laid-Open Patent Publication No. 62-205656 discloses a method for reducing the specific resistance of a thin film used as a lower electrode and an electrode line by mixing Ta with Mo (molybdenum). It is described that the specific resistance of the thin film can be reduced to 40 .mu..OMEGA.cm by this method. However, when Ta is mixed with Mo to form an alloy, Mo in the thin film is eluted during anodization of the lower electrode and the electrode line. The resultant oxide insulator film is not as fine-grained as the oxide film obtained by anodic oxidation of a thin film formed of only Ta.
Japanese Laid-Open Patent Publication Nos. 4-13861 and 5-47708 each disclose a method for reducing the specific resistance of a thin film used as a lower electrode and an electrode line by doping Ta with nitrogen, using Kr (krypton) gas mixed with nitrogen gas as the sputtering gas. It is described that the specific resistance can be decreased to 40 .mu..OMEGA.cm by this method.
If the specific resistance of the Ta thin film is excessively low, the specific resistance of a TaO.sub.x film formed by anodic oxidation of a surface of the Ta thin film is raised. As a result, the symmetry of the current-voltage characteristic of the MIM device in the positive levels of the voltage and in the negative levels of the voltage is lowered. The inventors of the present invention have found that, when the specific resistance of the Ta thin film is 40 .mu..OMEGA.cm or lower, such symmetry is too low to prevent generation of residual images on a display panel of the liquid crystal display device. Accordingly, in the case where a Ta thin film is used as the lower electrode of an MIM device, a specific resistance of the Ta thin film should not be low. By the methods disclosed in the above-mentioned three publications, the specific resistance of the Ta thin film is 40 .mu..OMEGA.cm, which is not preferable.
However, while forming the liquid crystal cell, the active matrix substrate and the counter substrate are pressed and attached to each other by a heat treatment of relatively high temperature (approximately in the range of 150.degree. to 200.degree. C.). As shown in FIG. 45, the nonlinearity of the MIM device is gradually reduced, as the time for the heat treatment increases. The deterioration of the nonlinearity of the MIM device can be remarkably observed, particularly in the case where a thin film of Ta having a .beta. structure (hereinafter, referred to as "a .beta.-Ta film") is used as a lower electrode.
The .beta.-Ta film has conventionally been used in various fields. The .beta.-Ta film is deposited by sputtering in an atmosphere of Ar (argon) gas using a pure Ta target with a purity of 99.99%. Namely, the .beta.-Ta film is deposited by a reactive sputtering method using a piece of target. In the case of using the .beta.-Ta film as the lower electrode of the MIM device, immediately after the MIM device is formed, the nonlinearity of the MIM device is satisfactory; however, since the active matrix substrate where the MIM device is formed is thermally treated as described above, after the formation of the MIM device, the nonlinearity thereof is remarkably deteriorated. Accordingly, the temperature for the heat treatment should be decreased. However, the active matrix substrate and the counter substrate are not satisfactorily attached to each other by heat and pressure under low temperature, thus reducing reliability of the liquid crystal display apparatus.
On the contrary, the specific resistance of the thin Ta film is conventionally reduced by doping nitrogen into the Ta thin film, which can prevent the deterioration of the nonlinearity of the MIM device. In generally, Ta thin film is disposed with nitrogen using (Ar+N.sub.2) gas or (Kr+N.sub.2) gas as the sputtering gas. Such a method is generally referred to as "reactive sputtering". In reactive sputtering, nitrogen reacts to Ta while sputtering and is then taken into the thin film. Accordingly, it is important that nitrogen gas should flow uniformly in the sputtering chamber. The inventors of the present invention have found that, when the flow rate ratio of nitrogen gas with respect to Kr gas exceeds around 4%, the amount of nitrogen taken into the Ta thin film is non-uniform. This problem is not solved by the present technology of operating the sputtering apparatus. Hereinafter, the study conducted by the inventors of the present invention will be described.